For the 4 and 8-ohm models, (Judging from what i believe your budget is, I dont think your going to be putting 12 ohms through your subs) the manufacturer says they need about 2.5 cubes a pop. I especially like how they do not mention whether the subs should be paired with ROUND ports or slot-style, as you have. They SHOW a round port. But mention nothing about slot-style. So, getting the volume and length figures from the manu, you might want to repost a question about your port style and length again to the forum to see if one of the big boys will help.
hopefully, they have just been caught up in the holiday bustle and havnt checked this post yet.
Heres what i show from the 12w0 PDF. (I hope youve seen it)
12W0-4 = 2.5'^3 ported volume. (thats net) External Dimensions 18"W x14"H x24"D
ID of port, (Inside Diameter) 4" (Assuming it is Round) and 13.6" long.
12W0-8 = All same stats as above, except = A 12" port length, still a 4" ID
They both mount 6" in from the exterior of the front face, and 11" diameter hole.
Some would say for the W0 that 3/4" MDF is overkill, and ID suggest 1/2" all around but use the 3/4" for the front face. But Its up to you.
So, You have your box built at volume 2.3'^3. I suggest you build the box as per the manufacturers recomendations, because it is the W0 and not another model type.
Even though, when you put the subs in your 2.5 cubes, you will lose a little volume due to the space the sub is taking up, but that wont be much, and will help. As for the tuning hertz.. I dont see any recommendations from JL's website, and again you will have to ask one of the big boys.
You could also try one of these babies..
http://www.jlaudio.com/enclosures/CS212RGW0.html